The present invention relates to a hydraulic control system for a loader type vehicle and particularly to an improved hydraulic system for raising and lowering the boom and controlling the attitude of a material handling unit supported on the:boom during raising and during lowering of the boom.
Loader type vehicles generally include a boom pivotally carried on the vehicle with a material handling unit pivoted on the free end of the boom. The boom is raised and lowered on the vehicle by a boom cylinder having fluid supplied thereto through a boom control valve and the material handling unit is pivoted on the end of the boom through a unit cylinder having fluid supplied thereto through a unit control valve. In the absence of any self-leveling function, it is necessary for the operator of the loader type vehicle to operate both the boom control valve and the unit control valve to maintain the material handling unit level while raising and lowering the boom. This operation is not only difficult but also requires close attention of the operator.
Hydraulic leveling systems have heretofore been made such as disclosed in U.S. Pat. No. 3,563,137, in which fluid exiting from the rod end of the boom cylinder during raising of the boom, is passed through a valve type flow divider that directs one portion of the inlet flow to the piston end of the unit cylinder while dumping a remaining portion of the inlet flow to drain, to level the unit during raising of the boom. This patent also disclosed an embodiment to which the boom control valve was modified to provide a boom-raise, unit-leveling position and a boom-lower, unit-leveling position, in addition to the customary boom-raise and boom lower positions. In that embodiment, the boom control valve was operative to pass fluid exiting from the piston end of the boom cylinder during lowering of the boom through a second flow divider valve arranged to direct one portion of the flow to the rod end of the unit cylinder and to dump the remaining portion to drain. Valve type flow dividers divide flow from an inlet into separate streams at two outlets in accordance with the pressure drop through two orifices and can operate reliably only when the pressure at the inlet is greater than the pressure at either outlet. During lowering of the boom, a portion of the fluid from the piston end of the boom cylinder is passed to the rod end of the bucket cylinder. The area of the piston end of the boom cylinder is usually very large as compared to the area of the rod end of the unit cylinder and, under some operating conditions, the pressure at the piston end of the boom cylinder is too low as compared to the pressure required at the rod end of the bucket cylinder, to reliably move the bucket when the latter is loaded.
U.S. Pat. No. 5,447,094 discloses a leveling circuit using a valve type flow divider for controlling leveling of the bucket during raising of the boom and a motor type flow divider for controlling leveling of the bucket during lowering of the boom. While this hydraulic leveling system operates during both raising and lowering of the boom, the motor type flow divider significantly increases the cost of the leveling system. Further, motor type flow dividers divide flow in a fixed ratio and are not adjustable to change the fixed ratio to accommodate different size boom and unit cylinders such as are used in different loader-type vehicles.
It is an object of the present invention to provide a hydraulic control system for controlling the orientation of the material handling unit on a boom, and which is operative in an automatic leveling mode to reliably control leveling of the unit during both raising and lowering of the boom.
It is another object of the invention to provide a hydraulic control system for controlling orientation of a material handling unit on a lift boom, and which can be configured for use in material handling vehicles that have boom and unit direction control valves connected in parallel or in series.
Another object of this invention is to provide a hydraulic control system in accordance with the foregoing object, and which is operative when lowering the boom to modulate flow from the boom cylinder in a manner to control the start and rate of boom descent to effect reliable leveling of the unit.
Another object of this invention is to provide a hydraulic control system for controlling the orientation of material handling unit supported on a boom in accordance with the foregoing object and which is adapted for use with boom control valves having a float position.
In the present invention, a hydraulic control system is provided for operating a boom cylinder to raise and lower a boom and for operating a unit cylinder to tilt the unit relative to the boom. The control system is operative in a self-leveling mode to control leveling of the material handling unit during both raising and lowering of the boom, using a proportional flow divider/combiner valve having a primary inlet and first and second secondary outlets. In the automatic leveling mode, when the boom control valve is moved to a boom-raise position, fluid from the boom control valve is supplied to a boom-raise chamber in the boom cylinder and fluid from a boom-lower chamber in the boom cylinder is passed to the primary inlet of the flow divider/combiner valve which divides the flow into a first stream that is passed to the tilt-down chamber of the unit cylinder and a second stream which is returned to the reservoir through the boom control valve, to level the unit during raising the boom. When the boom control valve is moved to a boom-lower position, fluid from the boom control valve is supplied to a tilt-up chamber in the unit cylinder and to the second secondary passage of the flow divider/combiner valve, and flow from a tilt-down cylinder is passed to the first secondary passage of the flow divider/combiner valve. The streams from the first and second secondary passages are combined in a preset ratio in the flow divider/combiner valve and supplied to the boom-lower chamber of the boom cylinder to level the unit during lowering of the boom.
It is desired to initiate leveling of the unit when the boom starts to move down to inhibit spilling of material from the unit. In a presently preferred embodiment, the control system includes a modulating valve that is pilot actuated between open and closed positions to control flow to and from the boom-raise chamber of the boom cylinder. A pilot network is provided to operate the modulating valve when the boom control valve is in the boom-raise position, to open and pass fluid to the boom-raise chamber, and to operate the modulating valve when the boom-control valve is moved to a boom-lower position, to modulate flow from the boom-raise chamber and inhibit descent of the boom until the fluid pressure in the tilt-up chamber in the unit cylinder is sufficient to level the unit while the boom is lowered. The pilot network is also operative when the boom control valve is moved to a float position, to pass fluid from the boom-raise chamber of the boom cylinder and allow the boom to float down.